[ewoks] complete conversion to ewoks structure

- complete signature to be `run`
- fix test
- orangecontrib: make inheritance to `Registered` instead of `OWEwoksWidget`
Because `OWEwoksWidget` enforce to use the defined ewokstaskclass which is not something we want for now (avoid task associated with gui, processes on threads...)
5 jobs for esrftaskgraph_prototype in 7 minutes and 41 seconds (queued for 2 seconds)
Name Stage Failure
failed
test:python3.7-stretch-pyqt5_pymca_larch Test
TOTAL                                                7930   6428    19%

=========================== short test summary info ============================
ERROR est/app/test/test_exec.py
ERROR est/test/test_ewoks.py
!!!!!!!!!!!!!!!!!!! Interrupted: 2 errors during collection !!!!!!!!!!!!!!!!!!!!
============================== 2 errors in 11.12s ==============================
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
test:python3.7-stretch-pyqt5_pymca Test
    from orangecanvas.scheme import readwrite
E ModuleNotFoundError: No module named 'orangecanvas'
=========================== short test summary info ============================
ERROR est/app/test/test_exec.py
ERROR est/test/test_ewoks.py
!!!!!!!!!!!!!!!!!!! Interrupted: 2 errors during collection !!!!!!!!!!!!!!!!!!!!
============================== 2 errors in 3.19s ===============================
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
test:python3.7-stretch-pyqt5_larch Test
    from orangecanvas.scheme import readwrite
E ModuleNotFoundError: No module named 'orangecanvas'
=========================== short test summary info ============================
ERROR est/app/test/test_exec.py
ERROR est/test/test_ewoks.py
!!!!!!!!!!!!!!!!!!! Interrupted: 2 errors during collection !!!!!!!!!!!!!!!!!!!!
============================== 2 errors in 14.40s ==============================
Cleaning up file based variables
ERROR: Job failed: exit code 1